With the rapid development of wireless communication, all wireless communication products are made light, thin, short and small in appearance in trend and in fashion so as to cater to the demand of consumers market. Meanwhile, the wireless communication product is required to provide various services; it means that more and more system modules and elements will be installed in the limited space of the wireless communication product. Hence, the space for installing the antenna will be compressed significantly.
Because the conventional monopole antenna and PIFA (planar inverted-F antenna) antenna usually require wide metal strips to achieve the required wide bandwidths for practical applications, the loop antenna with a narrow strip width becomes an attractive choice for the demand for smaller and multi-band antenna. For example, a loop antenna with multiple metal arms is disclosed in U.S. Pat. No. 7,265,726 B2 “Multi-band antenna”, and used in GSM, DSC, and UMTS mobile communication system as an internal mobile phone antenna for multi-band operation. Though a narrow metal strip is used for the loop antenna, the required wide bandwidth can be obtained. But in this former case, half-wavelength mode and one-wavelength mode of the conventional loop antenna are used. The half-wavelength mode is provided for GSM operation, which makes the antenna size difficult to be reduced. On the other hand, according to “Antenna and wireless communication devices” disclosed in No. US 20070268191 A1, the multi-band operation can also be achieved by using a matching circuit. Here, a new design of a coupled-fed multi-band loop antenna is disclosed. This design is different from the conventional loop antenna used in the mobile phone, which uses the half-wavelength loop mode as its first resonant mode. The antenna of the present invention uses the quarter-wavelength mode of the loop antenna as its first resonant mode. In this case, for application in the same operating band, the size of the antenna can be reduced by half. Compared with the conventional design of the internal mobile phone antenna, the design of the present invention is capable of saving more antenna occupied space to accommodate other associated elements, such as the loudspeaker or camera lens, and so on. The antenna of the present invention is designed in a manner of using a coupling feed, so that the quarter-wavelength mode of the loop antenna can be excited successfully with good impedance matching. Thus, the size of the antenna of the invention is only half of the conventional loop antenna. Besides, a matching component group can further be used to compensate for the large imaginary part of the half-wavelength and one-wavelength resonant modes of the loop antenna, so that these two modes can also have good impedance matching, thereby the antenna can cover four operating bands of GSM/DCS/PCS/UMTS and satisfy the demand for wireless communications.